The present invention relates to a single-phase lithium ferrite based oxide which is suitable as a cathode material (positive-electrode material) for lithium ion secondary batteries, a process for preparing the same and uses thereof.
In recent years, attention is directed to lithium ion secondary batteries for use as a secondary battery mounted on a portable device such as portable telephones, note-size personal computers or the like because of their high energy density. It is expected that this type of battery will be applied as a large-size battery for electric automobiles, power-load levelling systems and so on. In this situation, the importance of lithium ion secondary batteries is increasing.
The cathode material is closely related to battery performance such as working voltage of a battery (a difference between the redox potential of a transition metal in the cathode and the redox potential of anode element), and charge and discharge capacities (an amount of Li removable from or applicable to the cathode) so that presumably a demand for the cathode material will increase with an increase of a need for lithium ion secondary batteries.
Today lithium cobalt oxide (LiCoO2) is in use as a cathode material for lithium ion secondary batteries. However, LiCoO2 which contains a rare metal cobalt is one of the factors raising the costs of raw materials for lithium ion secondary batteries.
For example, lithium manganese oxide (LiMn2O4) is receiving attention for use as a cathode material which is inexpensive and substantially free from the resource problem. Now the lithium manganese oxide has been partly put to practical use.
Further it is desired to commercially provide cathode materials prepared from iron, i.e. a cheap metal element, for abundance of resources and low toxicity compared with manganese. For example, lithium ferrite (LiFeO2) has been investigated for possible use as a material for electrodes. However, when lithium ferrite (LiFeO2) is prepared from an iron source such as iron oxide and a Li source such as lithium carbonate by calcining them at a high temperature or hydrothermally treating them, the lithium ferrite can scarcely function for charging and discharging and lacks an activity for lithium secondary batteries (K. Ado, M. Tabuchi, H. Kobayashi, H. Kageyama, O. Nakamura, Y. Inaba, R. Kanno, M. Takagi and Y. Takeda, J. Eelectrochem. Soc., 144, [7], L177, (1997)).
On the other hand, lithium ferrite (LiFeO2) prepared from xcex1-NaFeO2 or FeOOH by H/Li or Na/Li ion exchange method has a flat charge potential in the vicinity of 4V, but has a discharge potential less than 3V. Thus LiFeO2 is lower in discharge potential by about 1V or more than LiCoO2. Consequently it is difficult to use LiFeO2 as a substitute for LiCoO2 (R. Kanno, T. Shirane, Y. Kawamoto, Y. Takeda, M. Takano, M. Ohashi, and Y. Yamaguchi, J. Eelectrochem. Soc., 143, [8], 2435, (1996), Y. Sakurai, H. Arai, S. Okada, and J. Yamaki, J. Power Sources, 68, 711, (1997), L. Guenne, P. Deniard, A. Lecerf, P. Biensan, C. Siret, L. Fournes, and R. Brec, Ionics, 4, 220, (1998), and Japanese Unexamined Patent Publications 1998-120421 and 1996-295518).
On the other hand, Fe-doped LiNiO2 and LiCoO2, which are iron-containing oxides, are reported to exhibit Fe3+/Fe4+ redox behavior at about 4V (C. Delmas, M. Menetrier, L. Crogurnnec, I. Saadoune, A. Rougier, C. Pouillerie, G. Prado, M. Grune, L. Fournes, Electrochimica Acta. 45, 243, (1999) and H. Kobayashi, H. Shigemura, M. Tabuchi, H. Sakaebe, K. Ado, H. Kageyama, A. Hirano, R. Kanno, M. Wakita, S. Morimoto and S. Nasu, J. Electrochem. Soc., 147, [3], 960, (2000)).
However, the foregoing cathode materials contain Co or Ni, i.e., rare metals, so that the low-cost feature of iron is meaningless. Further the redox of iron may have been induced by the redox of Co or Ni, and it is unclear whether iron spontaneously undergoes Fe3+/Fe4+ redox.
Whether the lithium ferrite based oxide can be put to practical use as a cathode material for lithium ion secondary batteries is determined by whether it has flat charge and discharge potentials in the vicinity of 4V which are attributed to Fe3+/Fe4+ redox. As described above, a technique has been scarcely established for preparing inexpensive and resource-saving lithium ferrite based oxides having flat charge and discharge potentials in the vicinity of 4V due to Fe3+/Fe4+ redox potential. Consequently the development of the technique is desired.
A principal object of the present invention is to provide a single-phase lithium ferrite based oxide which is suitable as a cathode material for lithium ion secondary batteries, a process for preparing the same and uses thereof.
The present inventors conducted extensive research to overcome the foregoing prior art problems and found the following. When lithium ferrite (LiFeO2) having a low discharge potential (3V or less) make a solid solution with a layered rock salt-type compound Li2-xMO3-y wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, the solid solution has charge and discharge potentials in the region of 4V, and the charge and discharge potentials correspond to Fe3+/Fe4+ redox potential. The present invention was completed based on this novel finding.
According to the present invention, there are provided single-phase lithium ferrite based oxides, cathode materials for a lithium ion secondary battery, processes for preparing the oxides (the solid solution) and a lithium ion secondary battery which are as follows.
1. A single-phase lithium ferrite based oxide having a layered rock salt-type structure, the oxide comprising lithium ferrite (LiFeO2) based solid solution with Li2-xMO3-y wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, 0xe2x89xa6x less than 2, 0xe2x89xa6yxe2x89xa61 such that the proportion of iron is 0.1xe2x89xa6Fe/(Fe+M)xe2x89xa60.9 wherein M as defined above.
2. The single-phase lithium ferrite based oxide as defined in item 1, wherein the lithium ferrite (LiFeO2) make a solid solution with Li2-xMO3-y such that the proportion of iron is 0.21xe2x89xa6Fe/(Fe+M)xe2x89xa60.75.
3. A cathode material for a lithium ion secondary battery, the material comprising a single-phase lithium ferrite based oxide having a layered rock salt-type structure, the oxide comprising lithium ferrite (LiFeO2) solid solution with Li2-xMO3-y wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, 0xe2x89xa6x less than 2, 0xe2x89xa6yxe2x89xa61 such that the proportion of iron is 0.1xe2x89xa6Fe/(Fe+M)xe2x89xa60.9.
4. The cathode material for a lithium ion secondary battery as defined in item 3, wherein the lithium ferrite (LiFeO2) make a solid solution with Li2-xMO3-y such that the proportion of iron is 0.15xe2x89xa6Fe/(Fe+M)xe2x89xa60.75.
5. A process for preparing a single-phase lithium ferrite based oxide having a layered rock salt-type structure, the oxide comprising lithium ferrite (LiFeO2) solid solution with Li2-xMO3-y wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, 0xe2x89xa6x less than 2, 0xe2x89xa6yxe2x89xa61 such that the proportion of iron is 0.1xe2x89xa6Fe/(Fe+M)xe2x89xa60.9, the process comprising the steps of adding an aqueous solution of a lithium compound in a molar ratio (Li/(Fe+M)) of from 1 to 3 relative to the other metals to a mixed aqueous solution containing a water-soluble compound containing at least one species selected from the group consisting of Mn, Ti and Sn, and a water-soluble iron compound or to a water-alcohol mixed solution to give a precipitate, or adding a specified amount of lithium hydroxide to said mixed aqueous solution or said water-alcohol mixed solution, evaporating the aqueous solution and the precipitate to dryness, and calcining the residue in an oxidizing atmosphere or in a reducing atmosphere.
6. The process as defined in item 5, wherein the lithium ferrite (LiFeO2) is solid-dissolved in Li2-xMO3-y such that the proportion of iron is 0.2xe2x89xa6Fe/(Fe+M)xe2x89xa60.75.
7. A lithium ion secondary battery produced using the cathode material for a lithium ion secondary battery as defined in item 3.
According to the present invention, there is provided a single-phase lithium ferrite based oxide having a layered rock salt-type structure, the oxide comprising lithium ferrite (LiFeO2) solid solution with Li2-xMO3-y wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, 0xe2x89xa6x less than 2, 0xe2x89xa6yxe2x89xa61 such that the proportion of iron is 0.1xe2x89xa6Fe/(Fe+M)xe2x89xa60.9. Hereinafter the lithium ferrite based oxide comprising LiFeO2 solid solution with Li2-xMO3-y may be referred to as xe2x80x9cFe-doped Li2-xMO3-yxe2x80x9d.
According to the invention, there is also provided a cathode material for a secondary battery, the material comprising a single-phase lithium ferrite based oxide having a layered rock salt-type structure, the oxide comprising lithium ferrite (LiFeO2) based solid solution with Li2-xMO3-y wherein M is at least one species selected from the group consisting of Mn, Ti and Sn, 0xe2x89xa6x less than 2, 0xe2x89xa6yxe2x89xa61 such that the proportion of iron is 0.1xe2x89xa6Fe/(Fe+M)xe2x89xa60.9.
The lithium ferrite based oxide of the invention and the cathode material for a secondary battery according to the invention have a layered rock salt-type structure as shown in FIG. 1. The structure of the lithium ferrite based oxide is similar to the structure of LiCoO2 which is most frequently used now as a cathode material for secondary batteries. FIG. 1 shows also, for comparison, the crystal structure of a layered rock salt-type LiFeO2. The lithium ferrite based oxide of the invention and the cathode material for secondary batteries according to the invention are characterized in that the iron ions partly occupy a transition metal-containing layer containing Fe, Li and M ions (FIG. 1).
The amount of iron ions in the lithium ferrite based oxide of the invention is about 10 to about 90% (i.e., 0.1xe2x89xa6Fe/(Fe+M)xe2x89xa60.9) based on the total amount of metal ions except Li. The lower limit of amount of iron ions (Fe/(Fe+M)) in the oxide is about 21%, preferably about 25%, more preferably about 30%, most preferably about 35%. The upper limit of amount of iron ions (Fe/(Fe+M)) in the oxide is about 75%, preferably about 70%, more preferably about 65%, most preferably about 60%.
The amount of iron ions in the oxide for the cathode material of the invention is about 10 to about 90% (i.e., 0.1xe2x89xa6Fe/(Fe+M)xe2x89xa60.9) based on the total amount of metal ions except Li. The lower limit of amount of iron ions (Fe/(Fe+M)) in the oxide is about 15%, preferably about 20%, more preferably about 25%, most preferably about 30%. The upper limit of amount of iron ions in the oxide is about 75%, preferably about 70%, more preferably about 65%, most preferably about 60%. The excessive amount of iron ions increases the amount of iron which does not participate in charging and discharging. Thus, the excessive amount is undesirable in terms of the characteristics of batteries. On the other hand, an excessively small amount of solid-dissolved iron ions are likely to result in too small charge and discharge capacities.
The value of x in Li2-xMO3-y may be in a positive range insofar as the value is in the range which retains the layered rock salt-type crystal structure. Nevertheless the value of x as close to 0 as possible is desirable from the viewpoint of charge capacity. The value of x is usually approximately 0xe2x89xa6x less than 2, preferably approximately 0 xe2x89xa6xxe2x89xa61, more preferably approximately 0xe2x89xa6xxe2x89xa60.5.
The value of y in Li2-xMO3-y is usually approximately 0xe2x89xa6yxe2x89xa61, preferably approximately 0xe2x89xa6yxe2x89xa60.5, more preferably approximately 0xe2x89xa6yxe2x89xa60.2.
The cathode material of the invention may contain a phase of impurity such as Li2CO3 in the range which would not seriously affect the charge and discharge characteristics.
There is no limitation on processes for preparing the single-phase lithium ferrite based oxide of the invention nor on processes for preparing the cathode material of the invention. For example, they can be prepared by conventional processes for preparing ceramics such as a hydrothermal reaction process, a calcining process and the like. However, conventional calcining processes comprising dry-blending an iron oxide and manganese, titanium or tin oxide with a lithium source such as lithium carbonate and calcining the blend are unlikely to induce homogeneous blending between iron and other metals, making it difficult to obtain a homogeneous sample. For this drawback, the conventional calcining processes can not produce 20% or more Fe-doped single-phase lithium ferrite based oxide. A novel calcining process capable of producing 20% or more Fe-doped single-phase lithium ferrite based oxide is described below by way of example, and then a producing process involving a hydrothermal reaction is described as an example of conventional processes for preparing ceramics.
A calcining process capable of preparing 20% or more Fe-doped single-phase lithium ferrite based oxide comprises the steps of adding a lithium compound such as lithium hydroxide in a molar ratio (Li/(Fe+M)) of from about 1 to about 3 relative to the other metals to a mixed aqueous solution containing a water-soluble compound containing at least one species selected from the group consisting of Mn (e.g., divalent, trivalent or tetravalent Mn), Ti (e.g., trivalent or tetravalent Ti) and Sn (e.g., divalent or tetravalent Sn) and a water-soluble iron compound (e.g., divalent or trivalent Fe), or to a water-alcohol (such as ethanol or methanol) mixed solution to give a precipitate, or adding a specified amount of lithium hydroxide or like lithium compound to said mixed aqueous solution or said water-alcohol mixed solution, evaporating the solution and the precipitate to dryness, and calcining the residue in an oxidizing atmosphere or in a reducing atmosphere.
Examples of the water-soluble compound containing Mn, Ti or Sn which is used in the calcining process of the invention are chlorides, nitrates, sulfates, acetates and hydroxides of these metals. Usable as the metal source are aqueous solutions of oxides of metals such as Mn, Ti or Sn in an acid such as hydrochloric acid. These materials for the metal source may be either an anhydride or a hydrate. The foregoing water-soluble compounds can be used either alone or in combination.
In the calcining process, the amount of lithium used as the raw material should be adjusted to a molar ratio (Li/Fe+M)) relative to the other metals. The value may range from about 1 to about 3, preferably about 1.5 to about 2.5.
The mixing ratio of the water-soluble compound containing at least one species selected from Mn, Ti and Sn and the water-soluble iron compound which are used in the calcining process is suitably determined according to the molar ratio of Fe/(Fe+M)(wherein M is at least one species selected from Mn, Ti and Sn) in the contemplated based oxide. Namely the value of Fe/(Fe+M) in the raw materials substantially corresponds with that of Fe/(Fe+M) in the contemplated based oxide.
The calcining conditions can be suitably selected according to the kind of compounds to be used as the metal source or the like. The calcination can be conducted, for example, in the atmosphere or like oxidizing atmosphere, or hydrogen-containing atmosphere or like reducing atmosphere. The calcining temperature is about 200 to about 1000xc2x0 C., preferably about 300 to about 800xc2x0 C. The calcining time is about 1 to about 100 hours, preferably about 20 to about 60 hours.
Optionally the product obtained by the calcination may be crushed and the obtained powder may be calcined again under the same conditions as above.
The above-mentioned novel calcining process can produce 20% or less Fe-doped single-phase lithium ferrite based oxide as well as 20% or more Fe-doped single-phase lithium ferrite based oxide.
The producing process utilizing a hydrothermal reaction is exemplified below. For example, a process is available which comprises the steps of adding an aqueous solution of alkali to an aqueous solution containing a water-soluble compound containing at least one species selected from the group consisting of Mn (e.g., divalent, trivalent or tetravalent Mn), Ti (e.g., trivalent or tetravalent Ti) and Sn (e.g., divalent or tetravalent Sn) and a water-soluble iron salt (e.g., divalent or trivalent iron salt) such as iron nitrate (III), or to a water-alcohol mixed solution to give a precipitate, and hydrothermally treating the precipitate together with a lithium compound in the presence of an oxidizing agent and potassium hydroxide at 100 to 400xc2x0 C.
Examples of the alkali to be used are lithium hydroxide, sodium hydroxide, potassium hydroxide and ammonia water. Examples of the lithium compound to be hydrothermally treated along with the coprecipitate are lithium hydroxide (either an anhydride or a hydrate), lithium chloride, lithium nitrate and the like.
The hydrothermal process employs a solution prepared by dissolving iron and manganese, titanium or tin salt in water, a water-alcohol solvent mixture or the like in a total concentration of about 0.01 to about 2 M, preferably about 0.1 to about 0.5 M (calculated based on anhydride). The molar ratio Fe/(Fe+M) is suitably determined according to Fe/(Fe+M) in the contemplated single-phase lithium ferrite based oxide.
While the above-mentioned mixed solution is stirred, an alkaline aqueous solution containing potassium hydroxide, sodium hydroxide or the like in a concentration of about 0.1 to about 20 M, preferably about 0.5 to about 10 M is added dropwise until the solution is rendered completely alkaline (preferably until the pH is adjusted to 11). After dropwise addition, aging treatment is performed at 0 to about 150xc2x0 C., preferably about 20 to about 100xc2x0 C. while blowing the air to obtain a precipitate. The obtained precipitate is washed with water to remove the excess alkali component and the residual salts. The precipitate is removed from the solution by filtration and dried at about 100xc2x0 C., thereby giving a coprecipitate. The coprecipitate is mixed with water in a vessel (e.g., polytetrafluoroethylene beaker). A lithium compound such as lithium hydroxide is added to the solution. The amount of the lithium compound to be added is determined such that the concentration of the compound in the solution is about 0.1 to about 10 M, preferably about 1 to about 8 M. An oxidizing agent such as potassium chlorate is added to the solution. The amount of the oxidizing agent to be added is determined such that the concentration of the agent in the solution is about 0.1 to about 10 M, preferably about 1 to about 5 M. The vessele containing the solution was left to stand in a hydrothermal reaction device (e.g., commercially available autoclave) to make a hydrothermal reaction. The reaction conditions are not limited but the reaction temperature is about 100 to about 300xc2x0 C., preferably about 150 to about 250xc2x0 C. The reaction time is not limited but preferably about 0.1 to about 150 hours, more preferably about 1 to about 100 hours. The hydrothermal reaction can be carried out, for example, in the atmosphere or like oxidizing atmosphere. After completion of the reaction, the reaction product may be optionally washed with water, filtered and dried to remove the excess residual salts and the like. In this way, the desired layered rock salt-type single-phase lithium ferrite based oxide is obtained. To improve the crystallizability of the sample, optionally the hydrothermally obtained sample may be mixed with a Li compound and the mixture may be calcined in the atmosphere or like oxidizing atmosphere. The calcining conditions may be the same as in the calcining process of the invention.
The single-phase lithium ferrite based oxide of the invention and the cathode material for lithium ion secondary batteries according to the invention can be applied to lithium ion secondary batteries by conventional methods. Useful anode materials are not limited and include, for example, metal lithium, carbon and the like. Useful electrolytes are not limited and can be suitably selected according to the upper limit of potential and the like. Examples of useful electrolytes include lithium perchlorate, LiPF6 and like lithium salts. Examples of useful solvents for the electrolyte are ethylene carbonate, dimethyl carbonate and the like.
According to the present invention, there can be obtained an inexpensive and high-capacity lithium ferrite based oxide useful as a cathode material for lithium ion secondary batteries.
Given below are Examples to further clarify the features of the present invention in more detail.
The crystalline phase of the samples obtained in the Examples was evaluated by X-ray diffraction analysis. The valence state of iron in the sample was evaluated by 57Fe Mxc3x6ssbauer spectroscopies, and the valence state of manganese was analyzed by X-ray absorption spectrum in MnK absorption edge. The composition of the sample was evaluated by inductively-coupled plasma (ICP) and atomic-absorption spectroscopy.
A coin-type lithium battery was produced using the sample as the cathode and metal lithium as the anode to investigate the charge and discharge characteristics of the battery.